drm/scdc-helper: Use consistent error reporting

The error messages generated by the SCDC helpers are somewhat
inconsistent with other DRM errors and even with other errors in the
same file. Fix them all up to use a common format.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901144430.19164-2-thierry.reding@gmail.com